As was reported before me (salesforcewatch, salesforce collective), the Winter release looks to be planned for Friday, January 12. I assume they will take that weekend to get it up, running and ready for Monday, the 15th.

The email I received states they will begin at 6:00pm pacific time on the 12th and end 2:00am on the 13th. I assume the exact times will be different for their different servers. The email I received was for NA3.